Units in grasshopper/Rhino

Hello.., Greetings!

Can anyone please suggest what is the unit of mass in Grasshopper? I specified ‘millimeter’ units in Rhino, and then entered a number in the ‘mass’ input of ‘particles from points’ command of “Flex” plugin in Grasshopper (as shown in picture below). I was wondering what would be the unit of mass here?

Many thanks .. !